Dispersions made by dispersing a liquid or solid dispersoid in a liquid dispersion medium are used for products such as foods, beverages, cosmetics, paints, fuel, and pharmaceuticals. To secure the quality of such products, dispersion stability is an important element.
Patent Document 1 describes a dispersion stability evaluation by comparing the turbidity of a dispersion made by dispersing asphaltene in a mineral oil with the turbidity of a sample solution obtained by centrifuging the dispersion. Patent Document 2 describes a method for evaluating the stability of a sample containing a particle dispersion body by forming particles in the dispersion body into a massive state and by detecting an increase in the massive-state particles. Patent Document 3 describes evaluation of redispersibility of cream contained in milk tea by visually observing a state when the milk tea is shaken. Patent Document 4 describes redispersion of drugs contained in an aqueous suspension eye drop. More specifically, Patent Document 4 describes an evaluation of redispersibility of drugs by rotating a container containing the aqueous suspension eye drop so as to redisperse the drugs in the aqueous suspension eye drop, and counting the number of rotations, which permit uniform redispersion of the drugs.
Techniques for analyzing the quality of dispersions are important for shortening the time of developing products using dispersions and stabilizing the quality of the products. In particular, since the state of an agglomerate of a dispersoid formed in a dispersion is deeply related to the dispersion state of the dispersion, its analysis is important. However, practical propositions as to a technique for quantitatively analyzing the state of an agglomerate of a dispersoid, and a quantitative evaluation technique based on the state of the agglomerate have not been made to date.
In the technique described in Patent Document 1, although it is possible to obtain findings as to how easily separation (precipitation) generated by the density difference between a dispersion medium and a dispersoid occurs, any findings as to the state of an agglomerate of a dispersoid cannot be obtained. In the technique described in Patent Document 2, although it is possible to obtain findings as to how easily particles agglomerate, any findings as to the state of an agglomerate of a dispersoid cannot be obtained. The technique described in Patent Document 3 poses a problem that it is difficult to apply this technique to dispersions in which dispersoid is difficult to be visually recognized. Furthermore, determination based on visual observation as described in Patent Document 3 may be inadequate for quantitative analysis technique required in product development, or analysis technique for determining the product quality. For the same reason, the technique described in Patent Document 4 may be inadequate as an analysis technique since a state in which drugs in an aqueous suspension eye drop are uniformly redispersed is determined by visual observation.